After import to Unity, all my normals are flipped
My guess would be that your object has negative scale in Object mode. As .obj does not store object transforms, the scale is applied on export and so causes the normals have negative direction as well.
To fix this, in Object Mode hit Ctrl + A  &gt; Scale, then do the operation described in answer you already found (in Edit Mode hit Ctrl + N or Shift + N for 2.8+) and then re-export.